Method and means for thermally stressing wire elements



Jan- 4, 1966 P. L. ELmoTT ETAL 3,227,848

METHOD AND MEANS FOR THERMALLY STRESSING WIRE ELEMENTS Filed March 9, 1964 f26a [le INVENTORS JOHN E RICHTER PAUL L ELL/OTT ATTORNEYS United States Patent Of 3,227,848 Patented Jan. 4, 1966 r. ICC

3,227,348 METHOD AND MEANS FR THERMALLY STRESSING WIRE ELEMENTS Paul L. Elliott, Pacifica, and John F. Richter, San Mateo,

Calif., assignors to Eitel-McCuliough, Inc., San Bruno,

Calif., a corporation of California Filed Mar. 9, 1964, Ser. No. 350,447 11 Claims. (Cl. 219-9.5)

This invention relates generally to a method and means for thermally stressing wire elements on a frame and more specifically relates to a method and means for thermally stressing grid wires on a frame to form a planar grid wire assembly for use in an electron tube.

In the past, a great deal of effort has been devoted to develop sturdy planar grid wire assemblies capable of sustaining7 elevated temperatures, such as are encountered in an electron tube environment, without having the wires sag or become distorted during tube operation.

Typically, a tungsten or molybdenum grid wire assembly utilized for electron tube application must be stressed to 50 or 60 percent of its ultimate tensile strength in order to allow for thermal expansion, at tube temperatures as high as 500 or 600 C., without sagging, Sagging or other distortions of the grid wire assembly cause deterioration of electrical characteristics, and/ or castastrophic malfunction.

Much of the work in this field has relied upon heating and stressing dissimilar materials which have different coefficients of expansion. the frame than for the wires permits the frame and the wires to be heated at one temperature and thus simplifies the stressing operation.

Some of the work in this field relies upon mechanical means to stress wires on a frame where both the wires and the frame are of the same material. Another method of stressing wires on a frame where both the wires and the frame were of the same material relied upon a differential cooling treatment, but mechanical gripping means were required to take up the slack in the wires during the heating step. Furthermore, the prior art techniques which used mechanical means in stressing the wires did not provide uniform or reliable tensioning of all the individual wires and therefore, they did not provide a quick or efficient method for stressing grid wires of a grid assembly for use in high temperature electron tube applications.

In addition, the temperatures obtained by past heating methods were not high enough to use pure gold as a brazing material which has a low secondary emission coefficient and hence, is very useful as a coating for grid Wires. The use of higher heating temperatures also permits brazes to be made at higher temperatures than could be previously achieved without relieving the stress in the wires.

It is an object of this invention to provide a method for thermally stressing wires on a frame.

It is a further object of this invention to provide a method for thermally stressing grid wires on a frame of the same material as the grid wires in order to form a planar grid assembly for use in an electron tube.

Briefly described, this invention relates to a method for thermally stressing wires on a frame. The temperature of the wires is raised to a higher temperature than the frame to create a temperature differential between the frame and the wires sufficient to expand the wires beyond the expansion of the frame. The frame is then cooled faster than the wires to increase the temperature differential between the frame and the wires. The frame is joined to the wires during the cooling period so as to permit the wires to be thermally stressed on the frame.

This invention also relates to an apparatus for thermal- Iy stressing wires on a frame. A base member is pro- Using a different material for vided to support the wires and the frame, An electrically conductive member is mounted on the wires. Energizing means are provided to cooperate with the electrically conductive member for creating and maintaining a temperaaturc differential between the wires and the frame for a period of time sufficient to thermally expand the wires beyond the expansion of the frame and to thermally stress the wires on the frame.

The other objects and important features of this invention will become apparent from the following specification and claims when read in connection with the figures of the drawing.

In the drawing:

FiGURE l is a cross-sectional View of one arrangement for thermally stressing grid wires on a frame to form a planar grid wire assembly in accordance with the method of this invention;

FIGURE 2 is a cross-sectional view of another arrangement for thermally stressing grid wires on a frame to form a planar grid wire assembly in accordance with the method of this invention; and

FIGURE 3 is a partial cross-sectional view of another type of grid wire mesh that can be used in the planar grid wire assembly made by the method of this invention.

Referring to FIGURE 1, a hollow dielectric base member 10, which is preferably movable, is positioned within a copper cylinder 12. The copper cylinder 12 has an annular inner protrusion 14 to concentrate the R.F. field that is generated by means of water cooled, conductive coils 16 which are helically wound about the cylinder 12. An AC. energy source 18 is electrically connected to the water cooled conductive coil 16 for generating the desired RF. field. Mounted on the dielectric base member .l0 is an interwoven grid wire mesh 20 which is to be thermally expanded and joined to a grid frame 22. The grid wire mesh 20 is preferably either of tungsten or molybdenum and the frame 22 is either a tungsten or molybdenum washer, respectively. Inserted between the frame 22 and the wire mesh 20 is a washer 24 of brazing material, such as gold. Also mounted on the grid wire mesh 20 is a plug 26 of conductive material, such as graphite which weighs approximately one-half of an ounce and is made of high density carbonl The plug 26 is only in contact with the portion of the grid wire mesh 20 located within the inner diameter of the frame 22.

In one example, the gold washer was l mil thick and the outer diameter of the frame 22 was l and 3/16 inches. The inner diameter of the frame was 11/16 of an inch.

The method of forming a thermally stressed, planar, grid wire assembly is as follows: The R.F. concentrator assembly which comprises the copper cylinder 12 and the annular inner protruding member 14 is energized by applying the A.C. energy from source 18 to the coil 16. The RF. concentrator assembly is used to heat the graphite plug 26 to a temperature of between 1200 to l250 C. This heating is continued for a period of approximately 7 seconds so that the temperature of the portion of the grid wire 20 in direct contact with the graphite plug is also in the range of between 1200 to 1250". The conductive heating of the grid wires 20 will cause them to be thermally stretched or expanded along the top surface of the base member 10. At the same time, the frame 22 and the washer 24 are indirectly heated to a temperature of about 1100 C. both by radiation from the graphite plug 26 and also by contact with the grid wire 20. The heating of the frame 22 and the washer 24 causes the gold brazing washer 24 to melt and the liquid gold flows across the entire grid wire mesh 20. The grid wire mesh 20, which is heated up quicker and to a higher temperature than the frame 22 of the washer 24, is thermally stretched or expanded beyond the thermal expansion of the frame 22 during the seven second heating period. The heating step is preferably carried out in an inert or reducing atmosphere, such as a hydrogen atmosphere.

At the end of the seven second heating period, the energy supplied to the RF. concentrator from source 18 is shut off to permit the frame 22 to be cooled. The frame 22 and the portion of grid wire mesh 20 in annular contact therewith cools quicker than the portion of the grid wire mesh 20 in contact with the heated graphite plug 26. The graphite plug 26 may be physically removed from direct contact with the grid wire mesh 20 when the frame 22 is joined to the grid wire mesh 20 upon the cooling and solidcation of the gold brazing material that. is between the frame 22 and the grid wire mesh 20. This is visually observed.

Consequently, the portion of the grid wire mesh 20 Within the inner diameter of the frame 22 is prevented from contracting to its fullest extent because the frame Z2 is joined to the outer portion of the grid wire mesh 2t). This method produces a thermally stressed, planar, grid wire assembly.

Referring to FIGURE 2, graphite plug 26a is provided with a protruding portion 28 which is in contact With the grid Wire mesh 20 and a recessed annular portion 30 so that during the heating period the frame 22, which is now located in Contact with the dielectric base member a, is indirectly heated by both the Wire mesh 20 and the reduced annular portion 30 of the graphite plug 26a. A space is provided between the grid wire mesh and the top surface of the base member 10a. This arrange ment permits the grid wire mesh 20 to be efficiently heated without the loss of heat from Contact with the solid base member 10a. However, to compensate for the sagging of the larger diameter grid wire mesh 20 during the heating period, a pair of annular metal portions 32 and 34 are tightly fixed to the outer portion of the grid wire mesh 20. The Weight of the pair of annular metal portions 32 and 34 is greater than the weight of the graphite plug 26a to compensate for any sagging in the grid Wire mesh 20. The method of thermally stressing the wire mesh 20 and joining it to the frame 22 is the same for both FIGURES l and 2.

FIGURE 3 is a cross-sectional view of a layer wound grid wire mesh 20a wherein the Wires are not interwoven but are composed of one parallel group of Wires perpendicular to another parallel group of wires. This grid wire mesh 20a can be substituted in place of the grid Wire mesh 20 of FIGURES l and 2.

Various modifications can be incorporated in this method of thermally stressing the wire elements 20 on the frame 22. The frame 22 can also be joined to the wire elements 20 by ultrasonic means While the frame 22 is being cooled. This would eliminate the need for the brazing washer 24.

In addition, to obtain a temperature differential between the frame 22 and the Wires 20, the frame 22 can be kept at a lower temperature than the wires 20 by the use of a suitable cryogenic apparatus. In this manner, the frame 22 will expand at room temperature which will cause stress in the wires 20 which are joined to the frame 22 during the rise in temperature of the frame 22.

What is claimed is:

1. A method for thermally stressing Wires on a frame comprising the steps of raising the temperature of the Wires higher than the temperature of the frame to create a temperature differential between said frame and said wires sufficient to expand said wires beyond the expansion of said frame, and then cooling the frame faster than the wires to increase the temperature differential between said frame and said wires, said frame being joined t0 said wires during said cooling step.

2. A method for thermally stressing wires on a frame comprising the steps of heating the wires by thermal conduction to a higher temperature than the frame to thermally expand the Wires beyond the expansion of said frame, and cooling said frame faster than said wires to increase the temperature differential between said frame and said Wires, said frame being joined to said Wires during said cooling step.

3. A method for thermally stressing Wires on a frame comprising the steps of mounting said wires and said frame on a base member, placing an electrically conductive plug in contact with a portion of said wires, applying RF. energy to said plug to conductively heat the portion of said wires in contact with said plug to thermally expand the wires beyond the expansion of said frame, said portion of said wires in contact with said plug being heated to a higher temperature than said frame, shutting off` the RF. energy being applied to said plug to permit said frame to cool faster than the portion of said wires in Contact with said heated plug, said frame being joined to said wires While said frame is being cooled, and cooling the remainder of said wires to thermally stress said Wires.

4. A method for thermally stressing grid Wires on a frame comprising the steps of placing the grid Wires on a lbase member, positioning a Washer member composed of brazing material on said grid Wires, placing the frame in contact with said brazing washer member, applying RF. energy to a graphite plug mounted only on a portion of said grid wires to conductively heat the portion of said grid wires to thermally expand said grid wires and to melt said brazing Washer material which ows along said grid Wires, shutting off the RF. energy being applied to said plug to permit the brazing material between said frame and said grid wires to solidify and join said frame to said grid wires, the portion of said grid wires within said frame in contact with said heated plug being kept at a higher temperature than said frame, and cooling said grid wires to thermally stress said grid wires on said frame.

5. A method for thermally stressing grid wires on a frame comprising the steps of positioning the frame on a base member, placing a Washer member composed of brazing material on said frame, applying R.F. energy to a graphite plug in contact with the portion of said grid wires within said frame to conductively heat said grid wires for a period of time sufficient to thermally expand said grid wires and to melt said brazing Washer material which iiows along said grid wires, said grid Wires having a weight connected to them which is heavier than said graphite plug so as to permit the heated grid wires to be tautly supported on said washer member, said graphite plug having a protruding portion in contact with said grid wires Within said frame and a recessed portion spaced from the portion of said grid Wires located on said frame, shutting 01T the R.F. energy being applied to said plug to permit the brazing material between said frame and said Wires to solidify and join said frame to said grid wires, the portion of said grid wires Within said frame in contact with said heated graphite plug being kept at a higher temperature than said frame, and cooling said grid wires to thermally stress said grid Wires on said frame.

6. A method for thermally stressing grid wires on a frame comprising the steps of mounting said Wires and said frame on a dielectric base member, placing a graphite plug in contact with a portion of said Wires, applying RF. energy to said graphite plug to raise the temperature of said plug to between 1200 to l250 C., conductively heating the portion of said grid wires in contact with said graphite plug for a period of about seven seconds to raise the temperature of said grid Wires to between 1200 to 1250 C. to thermally expand the grid wires beyond the thermal expansion of said frame, said frame being heated to a temperature of about 1100 C., shutting off the RF. energy being applied to said plug to permit said frame to cool faster than the portion of said grid wires in contact with said heated plug, said frame being joined to said Wires while said frame is being cooled, and cooling the remainder of said grid Wires to thermally stress said grid wires.

7. Apparatus for thermally stressing Wires on a frame comprising, in combination, a base member supporting the Wires and `the frame, an electrically conductive member mounted on said wires, and energizing means cooperating with said electrically conductive member for creating and maintaining a temperature differential between said Wires and said frame for a period of time sufficient to thermally expand said Wires beyond the expansion of said frame and to thermally stress said Wires on said frame.

8, Apparatus for thermally stressing wires on a frame in accordance with claim 7, in which said electrically conductive member mounted on said wires comprises a cylindrically shaped graphite plug.

9. Apparatus for thermally stressing wires on a frame in accordance with claim 7, in which said electrically conductive member mounted on said Wires comprises a cylin- 6 drically shaped graphite plug having a protruding portion at one end thereof which is in contact with the portion of said Wires located Within said frame.

10. Apparatus for thermally stressing Wires on a frame in accordance with claim 9, wherein weight means are attached to said Wires to keep said wires taut under the weight of said graphite plug.

11. Apparatus for thermally stressing Wires on a frame in accordance with claim 7, in which said energizing means comprises an RP. concentrator assembly.

References Cited by the Examiner UNITED STATES PATENTS 1,863,073 6/1932 Smythe 219-162 RICHARD M. WOOD, Primary Examiner. 

1. A METHOD FOR THERMALLY STRESSING WIRES ON A FRAME COMPRISING THE STEPS OF RAISING THE TEMPERATURE TO THE WIRES HIGHER THAN THE TEMPERATURE OF THE FRAME TO CREATE A TEMPEATURE DIFFERENTIAL BETWEEN SAID FRAME AND SAID WIRES SUFFICIENT TO EXPAND SAID WIRES BEYOND THE EXPANSION OF SAID FRAME, AND THEN COOLING THE FRAME FASTER THAN THE WIRES TO INCREASE THE TEMPERATURE DIFFERENTIAL BETWEEN SAID FRAME AND SAID WIRES, SAID FRAME BEING JOINED TO SAID WIRES DURING SAID COOLING STEP. 